A vinylidene fluoride polymer solution that is obtained by dissolving vinylidene fluoride polymer powder in N-methyl-2-pyrrolidone (hereinafter, also referred to as NMP) is used as a binder for lithium ion secondary battery.
In general, the binding force of vinylidene fluoride polymer powder serving as a binder increases with increasing molecular weight of the polymer. However, a polymer having a higher molecular weight requires a longer time to be dissolved in NMP, thereby deteriorating the productivity.
The reasons why the dissolution takes a long time include the facts that vinylidene fluoride polymer particles themselves become less soluble with increasing molecular weight and such vinylidene fluoride polymer particles adhere to each other in NMP so as to form large masses (hereinafter, also referred to as lumps).
In particular, because lumps prevent NMP from penetrating through the inside of lumps, the formation of lumps in NMP causes a very long time for vinylidene fluoride polymer powder to be dissolved in NMP.
A known method for dissolving a vinylidene fluoride polymer is to disperse vinylidene fluoride polymer powder in a poor solvent and thereafter stir the dispersion in a good solvent so as to dissolve the polymer (see, for example, Patent Literature 1). According to the method described in Patent Literature 1, acetone, tetrahydrofuran or the like is used as the poor solvent, and NMP or the like is used as the good solvent. The method disclosed in Patent Literature 1 is capable of dissolving a vinylidene fluoride polymer by a very simple technique. However, the method of Patent Literature 1 is complicated due to the need of dispersing vinylidene fluoride polymer powder in a poor solvent and thereafter stirring the dispersion in a good solvent, and tends to be unsatisfactory in terms of productivity. This literature also discloses an embodiment in which the poor solvent is removed from the vinylidene fluoride polymer solution. However, performing such a step of removing the poor solvent adds costs.
Porous vinylidene fluoride polymer powder is known to exhibit excellent solubility in NMP (see, for example, Patent Literature 2). The vinylidene fluoride polymer powder disclosed in Patent Literature 2 can be obtained by a supercritical suspension polymerization method having a step of suspending a vinylidene fluoride monomer and a step of performing supercritical polymerization. However, because the vinylidene fluoride polymer powder described in Patent Literature 2 is not particularly designed so as to prevent the formation of lumps, lumps can be formed to cause a decrease in solubility when the vinylidene fluoride polymer powder is dispersed in a solvent in an inappropriate manner.